Natural gamma-ray logging in the main hole (100-2000 m) of the Chinese Continental Scientific Drilling Project and its significance

Abstract：The Ground Core Gamma Eye was used to test the natural gamma values of cores (2000 m) in the main hole of the Chinese Continental Scientific Drilling (CCSD) Project. The peak and valley of the gamma curve and their corresponding petrological, structural and mineralization features were recorded. Through systematic logging, a 2000 m long natural gamma value profile was constructed. The gamma value of eclogite is low and gets high progressively from eclogite through amphibolite to granite gneiss. There is a distinct correspondence between the gamma values and the lithology of metamorphic rocks. Concentrations of the radioactive elements K, U and Th for different types of rock are varied. The genesis of metamorphic rocks is discussed in the context of the radioactivity of these elements in rocks. It is proposed that the genesis of rocks reflected by the protoliths of eclogites and meta-basic (-intermediate) rocks different from that of metapelitic sedimentary rocks. These rocks are orthometamorphites and high Th/U ratios of the rocks are related to the action of near-surface weathering or hydrothermal alteration. The systematic measurements of the natural gamma values of high-grade metamorphic rocks in the main hole are helpful to homing the depth and orientation of cores and studying the genesis of rocks, thermal state of the lithosphere and tectonic evolution.